The triple-cation mixed halide perovskite Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3emerges as one of the most promising candidates for photovoltaics due to superior optoelectronic properties, but the thermal stability is still a major challenge for the viability of perovskite solar cells towards commercialization. Herein, wefirstly explorethe thermal response of the photovoltaic performances to access device physical changes. It is shownthat the efficiency loss originates from decreased charge mobility, increased trap density and generation of PbI2charge recombination centers near the interface. In-depth analysis ofevolutions in morphology, chemical composition, dynamic and electronic structure of the perovskite layer at the nanometer scales indicates that it isinitial dangling bonds and vacancies on the imperfect surfaces decrease the activation energy and cause the perovskite decomposition in a layer-by-layer pathway sequentially from the film surface to bulk. Based on the results,astrategy of surfacepassivationtoimprove the thermal stabilityis demonstrated and discussed. This work for the first timeprovidesinsights into the physical and chemical change of such triple-cation perovskiteand indicatesthat more effortshould be invested in surface treatment for enhancing perovskitedevicestability.
